1. Field of the Invention
The present invention relates to diamond coated cemented carbide substrates used as inserts on cutting tools, which substrates have a surface susceptible to receive diamond coatings thereupon, and a method of preparing the surface of the cemented carbide substrate.
2. The Prior Art
For many years, the cutting tool art has struggled to find a repeatable way to utilize artificial diamond imbedded inserts on cutting tools to enhance the performance thereof.
Generally, artificial diamond is of two varieties, polycrystalline diamond (PCD) and chemical vapor deposition (CVD) diamond film. PCD cutting tools, comprising a piece of polycrystalline diamond fastened to the tip of a tool insert are known in the art. However, these tools are expensive to manufacture and do not readily lend themselves to indexing for increased tool life. In addition, PCD inserts for tooling having complex shapes, i.e., taps, drill bits, router bits and the like, cannot be formed using known techniques.
Consequently, numerous attempts have been made to provide diamond coated cutting tools using a chemical vapor deposition (CVD) process to deposit a coating or film upon a carbide substrate, such as tungsten carbide (WC), to provide a cutting tool with increased cutting performance. There has long been interest in inserts of CVD diamond-coated carbide substrate, because such an insert would be less costly to manufacture than a PCD insert, and because inserts of a CVD diamond-coated carbide substrate would be available in more complex shapes than with PCD tool inserts.
Sintered tungsten carbide (WC) substrates without cobalt (Co) or other binders have been studied but can be too brittle to perform satisfactorily as cutting tool inserts. A good discussion of the prior art may be found in U.S. Pat. No. 5,236,740, which discloses a technique for applying CVD diamond film to unpolished cemented carbide substrates. As explained therein, a significant challenge to the developers of diamond-coated tooling is to optimize adhesion between the diamond film and the substrate to which it is applied, while retaining sufficient surface toughness in the finished product. Cemented tungsten carbide substrates incorporating a cobalt (Co) binder in concentrations equal to or less than 6% (WC/Co) have the requisite toughness and thus show the greatest long-term commercial promise for tooling applications. A cemented tungsten carbide substrate with up to 6% cobalt would provide adequate surface toughness for most machining and cutting tasks. Cemented tungsten carbide can be formed into a variety of geometries, making it a potential material for cutting tool inserts. To date, however, a repeatable, reliable CVD diamond coated cemented carbide substrate with repeatable cutting performance has escaped the art.
It is therefore a goal to provide a cemented tungsten carbide substrate which may be effectively and reliably coated with a layer of CVD diamond film having adequate adhesion to the substrate for use as an insert on a machine or cutting tool. However, the solution to the problem of adequate, consistent and reliable adhesion of CVD diamond films to cemented carbide substrates has long eluded the art.
It has been reported in the literature that the use of a cobalt binder in cemented carbides inhibits adhesion of the diamond film to the substrate. R. Haubner and B. Lux, Influence of the Cobalt Content in Hot-Pressed Cemented Carbides on the Deposition of Low-Pressure Diamond Layers, Journal De Physique, Colloque C5, supplement no 5, pp. C5-169-156, Toma 50, May 1989. Indeed, conventional wisdom indicates that successful use of cemented rungsten carbide substrates may only be achieved by utilizing substrates containing no cobalt, as taught in U.S. Pat. No. 4,990,403; or no more than 4% Co binder, as taught in U.S. Pat. No. 4,731,296, or by deliberately depleting the cobalt concentration at the surface of the substrate. It is known to deplete the cobalt concentration at the surface of the substrate by selective etching or other methods, M. Yagi, Cutting Performance of Diamond Deposited Tool for Al 18 mass % Si Alloy, Abst. of 1st Int. Conf. on the New Diamond Sci. & Technol, pp. 158-159, Japan New Diamond Forum 1988. However, this decreases the surface toughness of the substrate and can cause chipping of the substrate and applied diamond film. Increased adhesion of diamond to the substrate may be achieved by decarburizing the substrate prior to deposition, as taught in European Patent Application Publication No. 0 384 011, but use of this procedure does not optimize substrate toughness and does not lend itself well to manufacturing environments where repeatability and consistency are important issues.
Some aspects of the prior art speculate that polishing or scratching the surface of a cemented tungsten carbide substrate prior to attempting diamond deposition may achieve improved adhesion of CVD diamond film, claiming there is an enhancement to the nucleation process caused by scratching and polishing. Haubner and Lux; Yagi; M. Murakawa et al., Chemical Vapour Deposition of A Diamond Coating Onto A Tungsten carbide Tool Using Ethanol, Surface Coatings Technology, Vol 36. pp. 303-310. 1988; Kuo et al. Adhesion and Tribological Properties of Diamond Films on various substrates, J. Mol. Res. Vol. 5, No. 11, November 1990, pp. 2515-2523. However, examination of these articles confirm that use of polished substrates yields poor results obtained by utilizing substrates whose surfaces have not been prepared by polishing or scratching.
Another attempted solution to the adhesion problem has been to employ an interlayer between the diamond and a WC/Co substrate. This encapsulates the Co, optimizing adhesion while allowing the substrate to retain its toughness. It may also be possible to choose an effective interlayer material that bonds strongly to diamond further increasing adhesion. U.S. Pat. No. 4,707,384 discloses use of a titanium carbide interlayer. U.S. Pat. Nos. 4,998,421 and 4,992,082 disclose utilization of a plurality of layers of separated diamond or diamond like particles interposed with layers of a planarized bonding material. However, to date these technologies have not been demonstrated to be commercially repeatable or viable.
Those skilled in the cutting tool art realize that the adhesion problem is particularly acute for cemented carbide cutting tools, and even more acute when the tool is intended to cut wood, especially man-made wood-like products such as particle board, fibre board and the like, because the carbide surface must be ground and/or polished to exacting dimensions to provide a keen or sharp (i.e., cutting) edge. Until now, no process or technique was effective or viable on a commercially repeatable basis for carbide substrates used for cutting tool inserts where the cutting edge of the carbide substrate was ground/polished. It has been discovered that the grinding/polishing process produces a film or "skin" on the surface of the cemented carbide substrate of undetermined morphology, which makes the adhesion of a CVD film, such as a diamond film, difficult and nonuniform, even when the substrate is acid-etched as the prior art teaches. It is believed that acid etching of cemented carbide surfaces is not effective on carbide substrates which have a "skin" because an etchant strong enough to penetrate/remove the "skin" also has the effect of corroding the carbide grains in a manner which renders the carbide grains ineffective as nucleating sites for the deposited CVD film. The present invention solves this long-standing problem, to provide a cemented carbide substrate very receptive to CVD diamond film, as shown by the cutting performance of inserts made according to the present invention.